There are two important groups of asbestos rocks: chrysotile and the amphibole. Both have similar properties, but they differ in terms of application and health risks.
Hydrous magnesium silicate, also called "serpentine", "white asbestos" or "chrysotile", does not represent any measurable risk to human health at exposures below 1 fiber/ml*. The fibrous structure is flexible, thin and silky and is rapidly eliminated by the respiratory system. It generally occurs in association with ultramafic rocks (dunite, peridodite, serpentinite) and can form large deposits, with reserves of the order of millions of tons.
At temperature above 800º C, chrysotile asbestos decomposes and is transformed into forsterite. This is a very important phenomenon since forsterite does not have a fibrous structure and therefore is innocuous to human health. Biopersistence studies on chrysotile asbestos (the time it remains in the lungs) have shown that the toxicity potential is very low because the molecule core contains magnesium, which means that it is biosoluble. These data have been confirmed by the leading Swiss toxicologist Dr. David Bernstein. In his studies, this eminent researcher demonstrated that the mean biopersistence of the chrysotile found in Brazil is 1.3 to 2.4 days in the lung tissue, a figure which is lower than the biopersistence of other industrial fibers.
Chemical composition: 3MgOSiO2H2O
High concentration of Magnesium
"Uma revisão da evidência recentemente publicada sobre riscos à saúde associados com tipo de fibra de amianto", de Jacques Dunnigan (PHD – Canadá – janeiro/99)
It is commonly found in nature and it occurs in association to several other rocks. Amphibole asbestos fibers propagate in the air more easily, they are not flexible and are not easily eliminated by the respiratory system. These fibers are hard, straight and spiky. They were widely used until the 70s, but are currently banned because of their harmful effect on human health. Even though they are found quite easily, this type of fiber does not occur in large or extensive deposits and as said, it is banned worldwide. This group of minerals has five main varieties:
Actinolite
Hydrous calcium magnesium iron silicate mineral: commercially speaking it is almost unknown, because deposits of its fibrous form are very scarce and difficult to find. The fibers are brittle and can be found in small quantities in amosite deposits.
Amosite (brown asbestos)
Hydrous iron magnesium silicate mineral: fibrous variety of grunerite-cummingtonite. The name amosite was formed from the first letters of the company Asbestos Mines of South Africa. Brown asbestos has glassy straight fibers, which are resistant to heat and mechanical forces and is also very elastic. The most important mines are found in South Africa.
Anthophyllite
Hydrous magnesium iron silicate: fibers are normally weak, short, and highly resistant to heat, acid and chemical substances in general. They are found in talc mines as an accessory mineral. The main deposits are located in Finland and in Brazil (Jiramataia/Alagoas).
Crocidolite (blue asbestos)
Hydrous sodium iron silicate mineral: fibrous variety of riebeckite, also known as blue asbestos. The fibers are straight and long, and intensely blue. It has a low fusibility point and is highly resistant to acids. Along with amosite, it is considered to have the best quality for commercial purposes. The main deposits are found in South Africa and Australia.
Tremolite
Hydrous calcium magnesium iron silicate mineral: The fibers are long and silky, with very little resistance to traction. They are found in chrysotile, and in talc deposits, as well as in other amphibole fibrous mineral deposits. Except for the Italian tremolite, it has very little commercial value.
Chemical composition: Na2OFe2O3OSiO2
High concentration of iron
Sources:
- Scliar, Cláudio. Amianto, mineral mágico ou maldito? CDI – Centro de Documentação e Informação LTDA, 1998.
- Amianto, muito além de saúde pública, Government of the State of Goiá
